This invention relates to a circuit arrangement for igniting and supplying a lamp with a DC current, comprising
input terminals for connection to terminals of a power supply source supplying a DC voltage, PA1 a DC-DC converter coupled to the input terminals and provided with PA1 an inductive element, PA1 a unidirectional element, PA1 a first switching element, PA1 a control circuit coupled to a control electrode of the first switching element for rendering the first switching element conducting and non-conducting at a frequency f, PA1 output terminals for connecting the lamp, and PA1 a first circuit which connects the output terminals during operation and comprises a second switching element and means for rendering the second switching element conducting and non-conducting. PA1 a light-transmissive discharge vessel having a filling comprising a rare-gas, and two electrodes, PA1 a lamp housing secured to the discharge vessel, PA1 a lamp cap having electric contacts and secured to the lamp housing, and PA1 a ballast circuit coupled between the contacts and the electrodes for generating a lamp current from a power supply voltage.
The invention also relates to a compact lamp.
A circuit arrangement as described in the opening paragraph is known from U.S. Pat. No. 5,581,161. In the known circuit arrangement, the DC-DC converter is constituted by a down-converter. The second switching element is conducting immediately after the known circuit arrangement is put into operation. The control circuit renders the first switching element conducting and non-conducting at a frequency f. During operation of the known circuit arrangement, both a first and a second lamp electrode form a part of the first circuit. The inductive element and the first circuit convey current during a first time interval, so that the electrodes of the lamp connected to the circuit arrangement are preheated. At the end of the first time interval, the second switching element is rendered non-conducting, so that the first circuit no longer conveys current. The inductive element subsequently generates an ignition voltage. After ignition of the lamp and during stationary operation, the control circuit renders the first switching element high-frequency conducting and non-conducting, and a lamp connected to the circuit arrangement is fed with a DC current supplied by the down-converter. Since the down-converter consists of only a small number of components, the known circuit arrangement can be manufactured relatively easily and thus also at a low cost.
The amplitude of the ignition voltage is dependent on the instantaneous amplitude of the current in the inductive element when the second switching element becomes non-conducting. In practice, the DC-DC converter is often operated in the discontinuous mode so as to limit switching losses. This means that the current through the inductive element in each period associated with the frequency f becomes substantially equal to zero during a given time interval. By rendering the first switching element conducting while the current in the inductive element is substantially zero, a considerable power dissipation in the unidirectional element is prevented so that the circuit arrangement has a relatively high efficiency. If the second switching element becomes non-conducting when the current in the inductive element is substantially zero or has a relatively low instantaneous amplitude, the energy present in the inductive element is insufficient to generate an ignition voltage with a sufficiently high amplitude. This problem could be solved by implementing the means for rendering the second switching element conducting and non-conducting in such a way that these means render the second switching element non-conducting when the instantaneous amplitude of the current in the inductive element has a relatively high value. Such an implementation of the means for rendering the second switching element conducting and non-conducting would, however, render the circuit arrangement relatively complicated and expensive.